Protection system for preventing defibrillation with incorrect or improperly connected electrodes

ABSTRACT

A protection circuit for a defibrillator that prevents a defibrillator pulse from being generated if the impedance between the defibrillator&#39;s electrode leads is not characteristic of the impedance between a pair of defibrillator electrodes properly connected to the defibrillator. The impedance measuring circuit applies a current to the electrode leads and the resulting voltage is measured to provide an indication of the electrode&#39;s impedance. The current is applied between the electrodes at about 33 kHz to approximate the impedance between a pair of defibrillator electrodes during a defibrillation pulse. The output of the measurement circuit is converted to an 8 bit word by an analog-to-digital converter and read by a microprocessor which compares the measured impedance to various impedance values in order to either generate an enable signal for the defibrillator or display messages indicative of open or short circuited electrode leads or a patient monitoring electrode connected to the electrode leads. the impedance measurement circuit operates in either of two ranges which are selected by the microprocessor on the basis of the measured impedance values.

DESCRIPTION

1. Technical Field

This invention relates to defibrillators that both defibrillate andmonitor through a common set of electrode leads and, more particularly,to a system for preventing defibrillation in the event that the leadsare connected to an incorrect electrode or are improperly connected to adefibrillation electrode.

2. Background Art

Defibrillators have long been used in the field of medicine to shock theheart into operating in a normal sinus rhythm when the heart goes intofibrillation. Defibrillators function by applying a relatively highpowered pulse at a relatively high voltage between a pair ofdefibrillation electrodes that are placed against the chest of apatient. The relatively high power, high voltage characteristics of thedefibrillation pulse require special electrodes that, among otherthings, provide a relatively large area of contact between theelectrodes and the patient. For this reason, defibrillation electrodesare relatively expensive, typically selling to the end user at between$10 and $20 each.

It is often difficult to determine whether a patient is in need ofdefibrillation. An examination of the patient's electrocardiogram (ECG)is often helpful in making this determination. Consequenly, manydefibrillators include an ECG monitor and associated circuitry to obtainan ECG of the patient. The ECG monitor is normally connected to a pairof patient monitoring electrodes that adhere to the chest of thepatient. The monitoring electrodes generate relatively low voltage,extremely low power, electrical signals indicative of the activity ofthe heart. The low voltage, low power characteristics of the signalsgenerated by the monitoring electrodes, in contrast to the high voltage,high power pulses delivered to defibrillation electrodes, makemonitoring electrodes substantially different from defibrillationelectrodes. Monitoring electrodes typically have a relatively small areaof contact with the patient, they are usually used once and thendisposed of and they normally cost about 50¢.

Monitoring electrodes are not suitable for use as defibrillationelectrodes for several reason. For example, the relatively small contactarea between the monitoring electrode and the patient would result inextremely high current densities if a defibrillation pulse was appliedto the monitoring electrode. The high current density would severelyburn the patient. Defibrillation electrodes are not normally used forpatient monitoring because of their relatively high cost.

When an emergency health care practitioner responds to a cardiacemergency, he or she normally obtains the patient's ECG to determine ifdefibrillation is necessary. ln many circumstances, defibrillation isnot required. Thus, in most circumstances, a patient monitoringelectrode is used, and the defibrillation electrodes are not required.If defibrillation is required, the health care practitioner must attachdefibrillation electrodes to separate electrode leads or they mustremove the patient monitoring electrode from the electrode leads andreconnect the defibrillation electrodes to the leads. Each of theseprocedures has a significant disadvantage.

The emergency conditions under which defibrillation normally occursmakes inadvertent defibibrillation through patient monitoring electrodesquite possible. As mentioned above, the practitioner will initiallyconnect the electrode leads to a patient monitoring electrode in orderto obtain an ECG. If an emergency condition, e.g., fibrillation, thenoccurs, it is quite possible for the practitioner to forget that thedefibrillator is connected to monitoring electrodes in the excitement ofthe emergency. In an effort to respond to the emergency as quickly aspossible, the emergency health care practitioner may cause thedefibrillator to generate a defibrillation pulse while the electrodeleads are still connected to the monitoring electrode.

The danger of defibrillating through patient monitoring electrodes iseven more acute because of the advent of automatic and semiautomaticdefibrillators used by relatively untrained personnel. Most cardiacemergencies occur outside the presence of trained health care personnel.Recognizing that defibrillation must occur very shortly after the onsetof fibrillation if it is to be successful, automatic and semiautomaticdefibrillators have been proposed in order to allow even untrainedpersonnel to defibrillate. In automatic or semiautomatic defibrillators,the patient's ECG is monitored and the defibrillator itself determinesfrom the characteristics of the ECG whether defibrillation is required.In the automatic defibrillator, the defibrillator automaticallygenerates a defibrillation pulse when defibrillation is required. In thesemiautomatic model, the defibrillator informs the practitioner thatdefibrillation is required. The practitioner then manually triggers thedefibrillator pulse. It is highly desirable in the case of asemiautomatic defibrillator, and absolutely required in the case of anautomatic defibrillator for the defibrillator to determine whether it isconnected to a defibrillation electrode. Obviously an automaticdefibrillator should not generate a defibrillation pulse if it isconnected to a monitoring electrode. While it is possible for anoperator of a semiautomatic defibrillator to ensure that thedefibrillator is connected to a defibrillation electrode prior tomanually generating the defibrillation pulse, the relatively untrainednature of semiautomatic defibrillation operators makes the likelihood ofmistake relatively high. Consequently, even for semiautomaticdefibrillators, it is highly desirable for the defibrillator to bedisabled from generating a defibrillation pulse if a patient monitoringelectrode is connected to the defibrillator.

In addition to the problems resulting from attempting defibrillationthrough patient monitoring electrodes, problems also exist whendefibrillation is attempted through improperly connected defibrillatoror patient monitoring electrodes. If the electrode leads areshort-circuited to each other when defibrillation is attempted, therelatively high current flow occurring through the short circuit candamage the defibrillator. More significantly, the short circuit preventsthe defibrillation pulse from reaching the patient, thus makingdefibrillation impossible. Likewise, if the electrodes are not properlyconnected to the electrode leads, an open circuit condition can existwhich also prevents the defibrillation pulses from reaching theelectrodes. Consequently, automatic and semiautomatic defibrillators arenot practical unless the defibrillator can determine for itself whetherthe defibrillator is properly connected to defibrillation electrodes.

DISCLOSURE OF THE INVENTION

It is an object of the invention to provide a defibrillator that may beused by relatively untrained personnel without the risk of attemptingdefibrillation through patient monitoring electrodes.

It is another object of the invention to provide a protective circuit toallow an automatic or semiautomatic defibrillator to both monitor anddefibrillate through common electrode leads.

It is another object of the invention to provide a defibrillator thatwill not attempt to defibrilate in the event that either an electrode isimproperly connected to the electrode leads of the electrode leads havebecome shorted.

It is still another object of the invention to provide an accuraterelatively inexpensive impedance measuring circuit for generating asignal indicative of the impedance between a pair of defibrillationelectrodes during a defibrillation pulse.

It is a further object of the invention to provide an impedancemeasuring circuit that is automatically switched to one of two operatingranges, depending upon the value of the measured impedance.

These and other objects of the invention are provided by a protectivecircuit for a defibrillator having an enable input allowingdefibrillation when an enable signal is applied to the enable input. Theprotective circuit includes an impedance measuring circuit connectedbetween the defibrillators electrode leads. The impedance measuringcircuit generates an output signal indicative of the impedance betweenthe electrodes. A processor receives the output of the impedancemeasuring circuit and generates the enable signal when the output signalis indicative of an impedance within a range of impedancescharacteristic of defibrillator electrodes properly connected to theelectrode leads. The impedance measurement circuit includes an ACcurrent source connected between the electrode leads so that the voltageacross the leads is proportional to the impedance between theelectrodes. The frequency of the AC signal is preferably about 33 kHz toapproximate the impedance between the defibrillator electrodes during adefibrillation pulse. In order to increase the range and sensitivity ofthe impedance measuring circuit, the circuit operates in one of tworanges, depending upon the measured impedance. In a low impedance range,the output of the impedance measuring circuit is a relatively largemultiple of the measured impedance. In the high impedance range, theoutput of the impedance measuring circuit is a relatively low multipleof the measured impedance. The processsor preferably includes ananalog-to-digital converter receiving the output of the impedancemeasurement circuit and generating a binary coded signal indicative ofthe measured impedance. The binary coded signal from theanalog-to-digital converter is applied to a microprocessor thatdetermines the impedance and generates the enable signal when theimpedance is within the proper range for defibrillation through anenable signal generating algorithm. The processor may also generate avisual message whenever the measured impedance is indicative of eitheran open circuit, a short circuit, a patient monitoring electrodeconnected to the electrode leads, or the impedance is between the rangesindicative of defibrillator electrodes or monitor electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a defibrillator employing the protectionsystem for preventing defibrillation when the defibrillator is eitherimproperly connected to electrodes or connected to the incorrectelectrodes.

FIG. 2 is a block diagram of an impedance measuring circuit used in theprotection system of FIG. 1.

FIG. 3 is a schematic of the impedance measuring circuit of FIG. 2.

FIG. 4 is a flow chart of the software used to program a microprocessorused in the protection system of FIG. 1.

FIG. 5 is a graph showing the operating ranges and range switchingpoints for the impedance measuring circuit of FIG. 3.

BEST MODE FOR CARRYING OUT THE INVENTION

A defibrillator incorporating the protective system is illustrated inFIG. 1. A defibrillator 10 is of the conventional variety having a pairof electrode leads 12, 14, connected to electrodes 16, 18, respectively.The electrodes 16, 18 are normally special purpose defibrillatorelectrodes when the defibrillator 10 is to be used to defibrillate apatient. However, the electrodes 16, 18 are normally patient monitoringelectrodes when the defibrillator 10 contains an ECG monitor and isbeing used to monitor the condition of the patient beforedefibrillation. In the event that the conventional defibrillator 10 doesnot include an enable input, an enable input can be easily implementedby one skilled in the art. For example, an appropriate semiconductorswitch can be placed in series or parallel with the normal, manuallyactuated switch of the defibrillator.

An impedance measurement circuit 20, described in greater detail below,is connected between the electrode leads 12, 14. The impedance measuringcircuit 20 applies an analog signal to a conventional analog-to-digitalconverter 22 indicative of the impedance between the electrodes 16, 18.The impedance measuring circuit 20 is able to determine the impedancebetween electrodes 16, 18 by measuring the impedance between the leads12, 14, since the impedance looking into the defibrillator 10 isnormally substantially greater than the impedance between the electrodes16, 18. The impedance measuring circuit 20 operates in one of tworanges, depending upon the condition of a two bit binary signal appliedto the impedance measuring circuit 20 by a properly programmedmicroprocessor 24. The microprocessor 24 receives an eight bit digitalword indicative of the impedance between the electrodes 16, 18 from theanalog-to-digital converter 22.

Although the microprocessor 24 may operate in a variety of modesdepending upon its programming, in the embodiment of FIG. 1 it isinterrupt driven each time a conventional timer 26 generates aninterrupt pulse which is applied to the interrupt port of themicroprocessor 24. When the microprocessor 24 is periodicallyinterrupted, it executes an interrupt subroutine that determines theimpedance indicated by the 8 bit word from the analog to digitalconvertor 22 and then switches the impedance measuring circuit 20 to theproper range, if required. If the impedance is within the range ofimpedances characterized by the impedance between a pair of properlyconnected defibrillation electrodes (i.e., normally 30-200 ohms), theimpedance measuring circuit 20 also applies an enable signal to theenable input of the defibrillator 10. If the impedance between theelectrode 16, 18 is within the range characteristic of a patientmonitoring electrode (i.e., normally 300-2,000 ohms), the microprocessor24 applies an appropriate signal to a message display device 28 toinform the operator that patient monitoring electrodes are connected tothe defibrillator 10. If the measured impedance is below the impedancethat is normally between a pair of properly connected defibrillationelectrodes (i.e., less than 30 ohms), the microprocessor 24 inhibits theunit from generating a defibrillation pulse. Finally, if the measuredimpedance is of a value normally characteristic of an open circuit(i.e., in excess of 2,000 ohms), the microprocessor 24 generates asignal causing the message display device 28 to inform the operator thatthe electrodes 16, 18 are not properly connected to the defibrillator10. The message display unit 28 may assume a variety of forms. In itsmost simple form, it may merely be three light emitting diodes each ofwhich are illuminated to display respective "open circuit," "shortcircuit," "patient monitor electrodes" and "defibrillation electrodes"messages.

A fourth mode exists between defibrillator and monitor electrodeimpedance ranges (200 ohms to 300 ohms) when the message "checkelectrodes" is enabled.

A block diagram of the impedance measuring circuit 20 is illustrated inFIG. 2. A 33 kHz sine wave oscillator 30 having a relatively high outputimpedance is connected between the electrode leads 12, 14. Since theoutput impedance of the oscillator 30 is substantially greater than theimpedance between the leads 12, 14 under all but open circuitconditions, the oscillator 30 functions as a 33 kHz current source. Thevoltage between the electrodes 12, 14 is thus representative of theimpedance between the electrode leads 12, 14, since the inter-electricalvoltage is equal to the product of the current (which is constant foreach range) and the inter-electrical impedance.

The oscillator 30 operates in either of two ranges, depending upon therange input applied to the impedance measurement circuit 20 by themicroprocessor 24 (FIG. 1). The use of two ranges allows the measurementcircuit 20 to achieve good sensitivity while measuring impedance over arelatively wide range as explained in greater detail below.

The electrode leads 12, 14 are also connected to an instrumentationamplifier 32 generating an output that is proportional to the voltagebetween the leads 12, 14 while providing good common mode rejection ofsignals applied equally to both of the leads 12, 14. The input impedanceof the instrumentation amplifier 32 is substantially greater than theimpedance between the electrode leads 12, 14 when they are connected toeven a patient monitoring electrode so that substantially all of thecurrent generated by the oscillator 30 flows through the leads 12, 14and the electrodes to which they are connected rather than through theinput terminals of the instrumentation amplifier 32.

The output of the instrumentation amplifier 32 is further boosted by a33 kHz amplifier 34 which increases the signal-to-noise ratio of thesignal by attenuating frequency components outside the 33 kHz frequencyrange generated by the oscillator 30. The output of the amplifier 34 isconverted to a DC voltage proportional to the magnitude of the AC signalat the output of amplifier 32 by a 33 kHz detector 36. The output of thedetector 36 is thus a DC signal having an amplitude that is proportionalto the impedance between the electrodes 12, 14.

The output of the instrumentation amplifier 32 is also applied to a 33kHz reject filter 38 that removes the 33 kHz frequency component fromthe signal at the output of the instrumentation amplifier 32. The outputof the 33 kHz reject filter 38 may be applied to a conventional ECGmonitor in order to monitor the condition of the patient prior todefibrillation. Since the defibrillator 10 (FIG. 1) is also connected tothe electrode leads 12, 14, it is apparent that both defibrillation andpatient monitoring occurs through the same electrode leads 12, 14.

As mentioned above, the use of multiple ranges in the impedancemeasurement circuit 20 increases the range and sensitivity of themeasurement circuit as compared to a single range instrumentationcircuit. Let us assume, for example, that the measurement circuit is tomeasure impedances between 30 ohms and 2 k and that the voltage at theoutput of the detector 36 is to vary between 0-10 volts. Under thesecircumstances, the output of the detector 36 with a measured impedanceof 2 kHz would be ten volts so that the sensitivity expressed in voltsper ohm would be 5×10⁻³. An impedance of 30 ohms would thus result in anoutput voltage of 0.15 volt. An impedance of 40 ohms would result in anoutput voltage of 0.2 volt. There would thus be only 0.05 voltseparating measured impedances of 30 and 40 ohms. By using two ranges,the output of the detector 36 can vary by ten volts between 30 and 200ohms and between 300 ohms and 2 k. Thus, in the low impedance range, thevoltage at the output of detector 36 will be 10 volts at 200 ohms, thusyielding a sensitivity of 5×10⁻² volts per ohm. A measured impedance of30 ohms would thus generate an output voltage of 1.5 volt. A measuredimpedance of 40 ohms would generate an output voltage of 2 volts. Thus,in the low range, the output voltage separating measured impedances of30 and 40 ohms would be 0.5 volt in contrast to the 0.05 volt separatingthe measured impedances of 30 and 40 ohms in the single range examplegiven above. It is thus seen that the use of two ranges provides greatersensitivity, yet still allows the impedance measuring circuit 20 tomeasure the same range of impedances. The above impedance and voltageexamples are provided merely for illustrative purposes, it beingunderstood that the principle remains the same regardless of the numberof measurement ranges, measurement sensitivities expressed in volts perohm, and output voltage limits that are used in the measurement circuit.

A schematic of the measurement circuit 20 illustrated in FIG. 2 is shownin FIG. 3. The oscillator 30 includes a conventional 33 kHz square waveoscillator 50 which may be of a conventional design such as an astablemultivibrator circuit. The oscillator 50 generates a 33 kHz square wavevarying between 0 and 5 volts. The output of the oscillator 50 isapplied to a pair of conventional solid state CMOS switches 52, 54through respective resistors 56, 58 and potentiometers 60, 62. Asexplained in greater detail below, the resistor 56 has a value ten timeslarger than that of resistor 58. Similarly, potentiometer 60 has a valuethat is ten times the value of potentiometer 62. As a result, whenswitch 52 is closed responsive to the high impedance range beingselected, the oscillator generates a signal having an amplitude that isten times smaller than when switch 54 is closed responsive to the lowimpedance range being selected.

The outputs of the switches 52, 54 are tied together and applied to thejunction of resistor 64, capacitor 60, and capacitor 68 which, incombination with feedback resistor 70 and operational amplifier 72,implement a high Q active band-pass filter centered at the 33 kHzfrequency of the signal generated by the oscillator 50. The outputs ofthe amplifier 72 are applied through an AC coupling capacitor 74 to theprimary 76 of a transformer 78. Capacitor 74 is provided to block DCoffsets at the output of amplifier 72 from flowing through the primary76 of the transformer 78 and thus saturating the core of the transformer78. A pair of diodes 80, 82 are provided to clamp the output of theamplifier 72 at approximately +5.7 volts and -5.7 volts to protect theamplifier 72 when a defibrillation pulse is applied to the electrodeleads 12, 14.

The high Q of the amplifier 72 causes a 33 kHz sinewave to be generatedat the secondary 90 of transformer 78 which is applied through couplingcapacitors 92, 94 and resistors 96, 98 to the electrode leads 12, 14.The resistors 96, 98 have a value that is substantially larger than theimpedance of even patient monitoring electrodes such as, for example, 20k ohms. Similarly, the coupling capacitors 92, 94 can have a relativelylarge impedance, such as 10 k ohms at 33 kHz. As a result, the currentflowing through the secondary 90 of transformer 78 is relativelyinsensitive to the impedance of the electrodes connected between theelectrode leads 12, 14, so that the oscillator 30 functions as an ACcurrent source.

The instrumentation amplifier 32 employs a pair of operationalamplifiers 100, 102 having their non-inverting inputs connected torespective electrode leads 12, 14 through respective resistors 104, 106.Resistors 108, 110 are connected to amplifiers 100, 102, respectively,to reference their inputs to ground. Diodes 112, 114, 116, 118 areprovided to clamp the inputs of the amplifiers 100, 102 at approximately+5.7 volts and -5.7 volts. Feedback resistors 120 and 122, incombination with resistor 124 set the gain of the amplifiers 100, 102 at2.

The outputs of the amplifiers 100, 102 are applied through respectiveresistors 130, 132 to the inputs of an operational amplifier 124 havingits gain set at unity by a feedback resistor 126. Resistor 128, incombination with potentiometer 130, is adjusted to equal the impedanceof resistor 126 to provide good, common mode rejection. The output ofthe amplifier 124 thus constitutes the output of the instrumentationamplifier 32 (FIG. 2).

As mentioned above, the output of the instrumentation amplifier 32 isapplied to a 33 kHz amplifier 34 and to a 33 kHz reject filter 38. The33 kHz amplifier 34 is implemented by two operational amplifiers 140,142 each having a lead network consisting of resistor 144 and capacitor146 for amplifier 140 and resistor 148 and capacitor 150 for amplifier142. Although the amplifiers 140, 142 have a gain of zero at DC, thegain of amplifier 140 is approximately 10 at the 33 kHz operatingfrequency. Similarly, amplifier 142 has a gain of approximately 10 asdetermined by feedback resistor 152 and resistor 148. Diode 156 isprovided to compensate for the forward drop of diode 160 in the detectorcircuit 36. Diode 158 prevents the output of amplifier 142 from swingingto the negative rail and causing reduced frequency responsecharacteristics. When the output of amplifier 142 is positive, currentflows through diode 160 and resistor 162 to charge capacitor 164positively. Resistor R154 causes a current to flow through diode 156equal to the current flowing through diode 160. This causes the forwardvoltage drops through both diodes to be approximately the same. Resistor166 is provided to allow capacitor 164 to discharge since the diode 160prevents the capacitor 164 from otherwise discharging. Capacitor 164thus charges to a value that is proportional to the peak amplitude ofthe 33 kHz signal applied to the 33 kHz amplifier 34. The gains of thevarious amplifiers are set so that the output of the 33 kHz detector 36is one volt when the impedance between the leads 12, 14 is 100 ohms inthe low range and 1,000 ohms in the high range. As a result, for a givenrange of voltages at the output of the detector 36, the impedancemeasurement circuit 20 can measure an impedance ten times greater in thehigh impedance range than in the low impedance range.

The 33 kHz reject filter 38 is required to remove a relatively large 33kHz signal from the relatively low amplitude ECG signal on the electrodeleads 12, 14. The output of the amplifier 124 is initially applied to aparallel tank circuit consisting of capacitor 170 and inductor 172 whichtogether have a relatively high parallel impedance at their 33 kHzresonant frequency. A series resonant circuit consisting of capacitor174 and inductor 176 has a relatively low series impedance at their 33kHz resonant frequency. As a result, capacitor 170 and inductor 172substantially block the 33 kHz signal and capacitor 174 and inductor 176shunt the remaining 33 kHz signal to ground. The resulting signal has amuch attenuated 33 kHz component.

A flow chart for controlling the operation of the microprocessor 24(FIG. 1) is illustrated in FIG. 4. As indicated above, themicroprocessor is periodically interrupted by interrupt pulses from aconventional timer 26. The microprocessor 24 then executes an interruptsubroutine that starts at 240. The program then determines at 242whether the impedance measurement circuit 20 is operating in the lowimpedance range. If so, the interrupt routine branches to 244 where the8 bit word from the A/D converter 22 is read and the indicated impedanceis compared to 30 ohms. If the measured impedance is less than 30 ohms,there may very well be a short circuit in the electrode leads 12, 14. Asa result, the microprocessor 24 outputs a message display bit at 246 toinform the user to check the electrodes. The interrupt routine thenreturns to main program or waits for the next interrupt at 248.

In the event that the impedance is found to be larger than 30 ohms at244, the routine branches to 250 where the measured impedance from theA/D converter 22 is compared to 200 ohms. If the measured impedance isless than 200 ohms, the impedance between the electrode leads 12, 14must be between 30 and 200 ohms. An impedance in this range ischaracteristic of the impedance between a pair of defibrillatorelectrodes that are properly connected to a defibrillator. For thisreason, the routine then generates the enable signal at 252 which isapplied to the enable input of the defibrillator 10 to allow thedefibrillator 10 to generate a defibrillator pulse.

In the event that the impedance is found at 250 to be greater than 200ohms, the routine branches to 254 where the measured impedance iscompared to 300 ohms. For the routine to reach step 254, the impedancemust, of necessity, be larger than 200 ohms. In the event that theimpedance is larger than 300 ohms, as determined at 254, themicroprocessor 24 switches the impedance measurement circuit 20 to theupper impedance range at 256. In the event that the measured impedanceis greater than 200 ohms, but less than 300 ohms, the impedance ischaracteristic of neither the impedance between a pair of defibrillatorelectrodes nor the impedance between a pair of patient electrodes. Forthis reason, the "check electrodes" message is generated at 246.

Assuming that the upper impedance range has now been selected, at thenext interrupt the subroutine will branch from 242 to step 258 where themeasured impedance is compared to 250 ohms. In the event that themeasured impedance is less than 250 ohms, the subroutine causes theimpedance measurement circuit 20 to be switched to the lower range at260. If the impedance is greater than 250 ohms, a comparison is made at262 to determine if the impedance is less than 300 ohms. If so, the"check electrodes" message is displayed at 246. If the impedance isgreater than 300 ohms, the measured impedance is compared to 2 k at 264.If the measured impedance is greater than 300 ohms but less than 2 k,the "monitor only" message is displayed at 268, since an impedance inthis range is characteristic of the impedance between a pair of patientmonitoring electrodes. If the impedance is found to be greater than 2 kat 264, it is likely that the electrode leads 12, 14 are open circuited,i.e., not properly connected to an electrode, so that the "checkelectrodes" message is once again displayed at 246.

The operating ranges and switch points for the microprocessor operatingaccording to the program illustrated in FIG. 4 are illustrated in FIG.5. When operating in the low impedance range, a measured impedance ofless than 30 ohms (i.e., less than 0.3 volt output from the impedancemeasuring circuit 20) causes a "check electrodes" message to bedisplayed at 246 (FIG. 4) since the electrode leads are probablyshort-circuited. Between 30 ohms and 200 ohms (between 0.3 volt and 2volts from the impedance measuring circuit 20) the enable signal isgenerated at 252 (FIG. 4) and is applied to the enable input of thedefibrillator 10 (FIG. 1). Between 200 and 300 ohms in either the low orthe high range, the check electrodes message 246 is displayed, but theimpedance measurement is not characteristic of either a short circuit,an open circuit, a defibrillator electrode or a patient electrode. Inother words, the results are indeterminate. When operating in the highimpedance range, a measured impedance of between 300 ohms and 2 k (i.e.,an output from the impedance measurement circuit 20 of between 0.3 voltand 2 volts) causes the "monitoring only" message to be displayed at268, since an impedance in this range is characteristic of the impedancebetween a pair of patient monitoring electrodes. A measured impedance ofgreater than 2 k (i.e., an output from the measurement circuit 20 ofgreater than 2 volts) is characteristic of an open circuit and thuscauses the "check electrodes" message to be displayed at 246.

In order to prevent the microprocessor 24 from alternately switchingbetween the upper and lower ranges when the impedance measurement is onthe borderline between two impedance ranges, the interrupt routine shownin FIG. 4 implements a hysteresis as illustrated in FIG. 5. When animpedance measurement of greater than 300 ohms is made while themeasurement circuit 20 is operating in the low impedance range, themicroprocessor 24 switches the impedance measurement circuit 20 to thehigh impedance range. Thereafter, the impedance measurement circuit 20will remain in the high impedance range even if the measured impedancedrops below 300 ohms. The impedance measurement circuit 20 will not beswitched back to the low impedance range until the measured impedance inthe high impedance range is less than 250 ohms.

It is thus seen that the inventive protective circuit prevents thedefibrillator 10 from generating a defibrillator pulse if either theincorrect electrodes are connected to the electrode leads 12, 14 or theelectrodes are improperly connected to the electrode leads 12, 14. As aresult, the defibrillator 10 can be safely operated by relativelyuntrained personnel and the defibrillator 10 can operatesemiautomatically or even automatically without risk of defibrillatingthrough patient monitoring electrodes or improperly connectedelectrodes.

We claim:
 1. In a defibrillator adapted to selectively monitor anddefibrillate electrodes, respectively, said defibrillator having anenable input allowing said defibrillator to defibrillate when an enablesignal is present on said enable input, a protection system forpreventing said defibrillator from defibrillating when saiddefibrillator is improperly connected to the correct electrodes througha pair of electrode leads or properly connected to the incorrectelectrodes through said electrode leads, said protection systemcomprising:impedance measurement circuit means connected between saidelectrode leads, said impedance measurement circuit generating an outputsignal indicative of the impedance between said electrodes; processormeans receiving the output signal from said impedance measuring circuitmeans and generating said enable signal when said output signal isindicative of an impedance within a range of impedances characteristicof a defibrillator electrode properly connected to said electrode leads;and range adjusting means for causing said output signal to have amagnitude that is proportional to a relatively large multiple of saidimpedance in a low impedance range and to a relatively small multiple ofsaid impedance in a high impedance range, thereby increasing the rangeof said impedance measurement circuit for a predetermined sensitivityand range of output voltages.
 2. The protection system of claim 1wherein said processor means includes range control means controllingthe operation of said range adjusting means, said range control meanscausing said impedance measurement circuit means to initially measurethe impedance between said electrodes in said low range and switchingsaid impedance measuring circuit to said high range when said outputvoltage is a value indicative of an impedance larger than a firstpredetermined value.
 3. The protection system of claim 2 wherein saidrange control means switches said impedance measurement circuit meansfrom said high range to said low range when said output voltage is avalue indicative of an impedance smaller than said first predeterminedvalue.
 4. The protection system of claim 3 wherein said range controlmeans switches said impedance measurement circuit means from said highrange to said low range when said output voltage is a value indicativeof an impedance smaller than a second predetermined value, said secondpredetermined value being substantially less than said firstpredetermined value, thereby providing hysteresis between the impedancethat said impedance measurement circuit means switches from said lowrange to said high range and from said high range to said low range. 5.A defibrillator adapted to both defibrillate through a defibrillationelectrode and monitor a patient through a monitoring electrode, saiddefibrillation electrodes and said monitor electrodes being connected tothe same pair of electrode terminals of said defibrillator, saiddefibrillator comprising:high voltage power supply means selectivelyapplying a high voltage defibrillation pulse between said electrodeterminals to defibrillate said patient; monitoring circuit meansreceiving an ECG signal from said patient between said electrodeterminals; disable means for preventing said high voltage power supplymeans from applying said high voltage defibrillator pulse between saidelectrode terminals whenever an enable signal is not being applied tosaid disable means; an AC current source connected between saidelectrode terminals so that the voltage of an AC signal generatedbetween said terminals is proportional to the impedance of the electrodeconnected between said terminals; detector means connected between saidelectrode terminals, said detector means generating a DC voltage that isproportional to the amplitude of the AC signal between said electrodeterminals; range adjusting means for causing said AC signal to have amagnitude that is proportional to a relatively large multiple of saidimpedance in a low impedance range and to a relatively small multiple ofsaid impedance in a high impedance range; and processor means receivingsaid binary coded signal, comparing the voltage of said binary codedsignal to a range of acceptable values indicative of the impedance of apair of defibrillation electrodes properly connected to said electrodeterminals and to said patient, and generating said enable signal at anoutput port if said binary coded signal has a value within said range ofacceptable values, said processing means further including range controlmeans controlling the operation of said range adjusting means, saidrange control means causing said impedance between said electrodeterminals in said low range and switching said impedance measuring meansto said high range when said AC signal has a voltage indicative of animpedance larger than a first predetermined value.
 6. The defibrillatorof claim 5 wherein said range control means switches said AC signal fromsaid high impedance range to said low impedance range whensaid AC signalhas a voltage indicative of an impedance smaller than said firstpredetermined value.
 7. The defibrillator of claim 6 wherein said rangecontrol means switches said AC signal from said high impedance range tosaid low impedance range when said AC signal has a voltage indicative ofan impedance smaller than a second predetermined value, said secondpredetermined value being substantially less than said firstpredetermined value, thereby providing hysteresis between the impedancethat said AC signal switches from said low impedance range to said highimpedance range and from said high impedance range to said low impedancerange.